1. Related Patent Application
This application is related to application (Docket No. PD-86484) Ser. No. 07/081,950, filed Aug. 5, 1987, and now abandoned concurrently with this application by Wei-yu Wu and assigned to Hughes Aircraft Company, the assignee of this invention.
2. Field of the Invention
This invention relates to methods for etching first and second types of semiconductor material to prepare structures of alternating ultrathin semiconductor layers, and in particular to forming selective electrical contacts on these structures. 3. Description of Related Art
The field of photonics combines laser physics, electro-optics, and nonlinear optics. An example of a photonic system is found in lightwave communications, where optical signals are generated, modulated, transmitted, and detected before they are changed into electrical form for ultimate use. Information processing provides another example of the application of photonics. There are several advantages of optical processing of information over electrical processing, which is limited in speed by pulse broadening in interconnected wires and in density by crosstalk between wires. Only the difficulty in developing convenient digital optical logic elements with low switching energy prevents the realization of optical systems having the capacity to handle extremely large quantities of information.
An ideal material for electro-optic applications would be able to transform current into light and vice versa for emission and detection. This ideal material would also display large optical and electronic nonlinearities which would permit its utilization as an optical gate and transistor. The material could be used for optical modulation by exploiting both nonlinearities simultaneously.
Semiconductor structures consisting of stacks of ultrathin layers are called superlattices or quantum-well structures. There are an enormous number of potential applications in photonics for devices made from these structures. The compound III-V semiconductors, which are made from elements of Groups III and V of the periodic table, are well suited for the fabrication of quantum-well structures. In these materials the energy band gap is direct, which means that light can be emitted or absorbed without the aid of lattice vibrations, in a very efficient process. Charge carrier mobilities are very large in these materials, and they are easily doped with impurities. From the structural point of view, it is very important that these materials can form solid solutions of various proportions with identical crystal structures and well-matched lattice parameters, but with different energy bandgaps and indices of refraction.
Superlattices and quantum-well structures are fabricated using the methods of molecular beam epitaxy (MBE) and metal-organic vapor deposition. These techniques make use of an ultraclean environment in combination with a slow growth rate to produce epitaxially grown materials. Junctions between different semiconductor materials (heterojunctions) can be made which are planar and atomically abrupt. Using growth rates as low as 1 Angstrom/second, it is possible to fabricate layered structures with layer thicknesses in the range from a few Angstroms to a few micrometers. Such multilayered structures display new properties not shown by the bulk semiconductor compounds.
One type of superlattice is the "doping superlattice," which is obtained by periodically alternating n and p doping during the growth of an otherwise uniform semiconductor such as gallium arsenide (GaAs). Originally the fabrication of doping superlattices with intrinsic layers in between the doped layers gave rise to the designation of "n-i-p-i" crystals. Doping superlattices in semiconductors provide modulation of the energy bands characterized by peaks and valleys. Holes sit on top of the peaks and electrons sit in the valleys. Electrons and holes are effectively spatially separated and confined. Characteristic series of energy subbands are created. Quantization of the carrier motion in the direction perpendicular to the layers produces a set of discrete energy levels. The effective energy gap can be set between zero and the gap of the host material by appropriately choosing the doping concentrations and thicknesses of the constituent layers. Also, the energy gap can be tuned by injecting carriers electrically and optically. Doping superlattices exhibit a variety of interesting new properties, such as extremely long lifetimes for electron-hole recombination, tunable electron and hole conductivities, very large photoconductive response, tunable absorption, tunable luminescence, and tunable optical gain.
A basic discussion of the n-i-p-i structure is given in an article by Klaus Ploog and Gottfried H. Dohler, "Compositional and Doping Superlattices in III-V Semiconductors", Advances In Physics, Vol. 32, No. 3, 1983, pages 285-359. This article presents a general discussion of n-i-p-i structures, as well as the spatial control of optical absorption by a voltage pattern applied to the n-i-p-i structure. Other applications, such as in photoconductors, photodiodes, ultrafast photodetectors, light emitting devices, and optical absorption modulators, are discussed in an article by Dohler, "The Potential of n-i-p-i Doping Superlattices For Novel Semiconductor Devices", Superlattices and Microstructures, Vol 1, No. 3, 1985, pages 279-287. A further expansion on n-i-p-i applications is provided in another article by Dohler, "Light Generation, Modulation, and Amplification by n-i-p-i Doping Superlattices," Optical Engineering, Vol. 25, No. 2, February, 1986, pages 211-218.
In order to exploit the unique electro-optical properties of n-i-p-i crystals it would be desirable to find a way of applying separate contacts to the sets of n and p layers of these structures. The fabrication of such so-called selective contacts has been attempted in different ways, but without success at doping levels high enough (at least 5.times.10.sup.18) to get sufficient energy band modulation.
Horikoshi and Ploog describe one attempt to fabricate selective n-i-p-i contacts in their article "A New Doping Superlattice Photodetector" in Applied Physics A37, pp. 47-56, published in 1985. The Method consists of providing n+ and p+ regions extending perpendicular to the layers on the two far edges and alloying small Sn and Sn/Zn balls to these regions to form the selective electrodes. The material in the balls is diffused through the layers by heating, and the heat required tends to move the dopant atoms in the layers. The blocking junctions tend to be leaky. Another attempt is described by Kuenzel et al. in Applied Physics Letters 38, at page 285, published in 1981. It makes use of evaporated and subsequently annealed Ni/Sn and Au/Zn contacts on etched n-i-p-i mesas, grown on undoped substrates. Neither of the techniques described above yields satisfactory results for GaAs n-i-p-i structures with doping levels greater than about 10.sup.17 cm.sup.-3. At doping concentrations significantly above 10.sup.18 cm.sup.-3 the contacts have turned out to be hardly selective at all in GaAs. The situation is even worse for n-i-p-i crystals made from lower bandgap material such as InGaAs.
An improved but more complicated method of making selective contacts is described by Doehler et al. in their article "In situ grown-in selective contacts to n-i-p-i doping superlattice crystals using molecular beam epitaxial growth through a shadow mask" in Applied Physics Letters 49, pp. 704-706, published in 1986. The method is restricted to n-i-p-i structures made by molecular beam epitaxy techniques. It makes use of atomic or molecular beams having different angles of incidence onto the substrate. The donor or acceptor beams are shadowed in certain parts of the regions where high-quality bulk material is growing. A silicon wafer with rectangular windows etched through it is placed on top of the GaAs wafer as a shadow mask. The mask is oriented such that the dopant beams are incident at equal angles with respect to the windows. When an n-p-n-p superlattice is being grown, on one side an n-i-n-i structure results and on the other a p-i-p-i structure. Due to the finite shadow width a laterally graded p-n junction barrier is grown in for each contact to its oppositely doped layer. Since practically no holes are present in the n-i-n-i regions, the only requirement for the metal contact to be applied to this structure is good ohmic behavior to n-type material. The corresponding requirement applies to the p-type metal contact. The disadvantages of this technique are that it requires special molecular beam epitaxy equipment, that only one device per wafer can be made, that it does not yield sharp boundaries at the contacts, and that it cannot be used for n-i-p-i structures made by liquid-phase epitaxy or metal-organic vapor deposition.